The present invention relates generally to RAKE receivers for code division multiple access (CDMA) systems, and more particularly, to a method and apparatus for determining the finger placement of RAKE fingers in a RAKE receiver.
In a wireless communication system, a transmitted signal may travel through multiple propagation paths so that the received signal is a composite of multiple time-shifted versions of the signal. The different time-shifted versions of the received signal, referred to herein as signal images, suffer from different phase and attenuation effects. The multiple time-shifted signal images combine at the receiver in an unpredictable manner resulting in signal fading.
CDMA receivers typically employ a RAKE receiver to combat signal fading due to multi-path propagation. The goal of the RAKE receiver is to detect the individual signal images and combine them coherently. A RAKE receiver typically includes a plurality of correlators, sometimes referred to as fingers, to separately despread different time-shifted signal images, and a combiner to combine the correlator outputs. For example, a RAKE receiver may detect and combine the M strongest signal images. A delay searcher processes the received signal to identify the delays corresponding to the strongest signal images, and a finger placement processor determines the finger placement based on those delays. The process of finger placement comprises the assignment of a delay to each RAKE finger to align the RAKE finger in time with a signal image. A simple finger placement strategy is to assign the delays of the J strongest signal images found by the delay searcher to respective RAKE fingers.
Typically, finger placement starts by generating an estimated power delay profile (PDP) over a defined search window that gives the signal power as a function of delay. An exemplary PDP is shown in FIG. 1. The delay searcher measures the signal power of the received signal samples. The spacing between samples defines a search grid and the signal power measurements define the PDP. One approach to finger placement, referred to herein as the “peak” approach, is to place fingers at or near the peaks or local maximas in the PDP. Ideally, the RAKE fingers would be placed at the exact delays corresponding to peaks in the PDP. As shown in FIG. 1, exact placement of the RAKE fingers at the peaks of the PDP is not always possible because the search grid does not always align with the peaks in the PDP. The actual peak in the PDP may fall between the grid points of the search grid.
The alignment of the search grid with the actual channel delay(s) affects performance. To illustrate this point, consider a simple flat (one-tap) channel. Upon reception of such a signal, a CDMA receiver attempts to correlate a time-shifted version of the spreading code with the received signal. The time-shift (delay), given by the searcher, is a function of the search grid resolution. If the grid is aligned with the channel delay, the contribution of all other codes is orthogonal, so there is no interference. If the grid is not aligned with the channel delay, then the orthogonality property is lost, and the contribution of other codes shows up as interference. The impact of misalignment on performance can be dramatic at very high data rates when higher order modulation such as 16-QAM is employed, as this modulation is highly sensitive to self-interference. Thus, when the path delay falls between two sampling intervals, the performance degradation can be severe. Consequently, data throughput may fluctuate between high data rates when the finger delays assigned to the RAKE fingers correspond to the path delays of the signal images, and much lower data rates when the finger delays assigned to the RAKE fingers do not align with the path delays.